In recent years, accompanying the adoption of multimedia information in cellular mobile communication systems, it has become common to transmit not only speech data but also a large amount of data such as still image data and moving image data. Furthermore, studies have been actively conducted in LTE-Advanced (Long Term Evolution Advanced) to realize high transmission rates by utilizing broad radio bands, Multiple-Input Multiple-Output (MIMO) transmission technology, and interference control technology.
In addition, taking into consideration the introduction of various devices as radio communication terminals in M2M (machine to machine) communication and the like as well as an increase in the number of multiplexing target terminals due to MIMO transmission technology, there is a concern regarding a shortage of resources in a mapping region for PDCCH (Physical Downlink Control Channel) that is used for a control signal (that is, a “PDCCH region”). If a control signal (PDCCH) cannot be mapped due to such a resource shortage, downlink data cannot be assigned to the terminals. Therefore, even if a resource region in which downlink data is to be mapped (i.e., a “PDSCH (Physical Downlink Shared Channel) region”) is available, the resource region may not be used, which causes a decrease in the system throughput.
As a method for solving such a resource shortage, a study is being made of assigning, in a data region, control signals for terminals served by a radio base station apparatus (hereunder, abbreviated as “base station”). A resource region in which control signals for terminals served by the base station are mapped is referred to as an Enhanced PDCCH (ePDCCH) region, a New-PDCCH (N-PDCCH) region, an X-PDCCH region or the like. Mapping the control signal (i.e., ePDCCH) in a data region as described above enables transmission power control on control signals transmitted to a terminal near a cell edge or interference control for interference by a control signal to another cell or interference from another cell to the cell provided by the base station.
Further, according to the LTE-Advanced system, in order to expand the coverage area of each base station, relay technology has been studied in which a radio communication relay station apparatus (hereunder, abbreviated as “relay station”) is installed between a base station and radio communication terminal apparatuses (hereunder, abbreviated as “terminals”; may also be referred to as UE (user equipment)), and communication between the base station and terminals is performed via the relay station. The use of relay technology allows a terminal that cannot communicate with the base station directly to communicate with the base station via the relay station. According to the relay technology that has been introduced in the LTE-Advanced system, control signals for relay are assigned in a data region. Since it is expected that the control signals for relay may be extended for use as control signals for terminals, a resource region in which control signals for relay are mapped is also referred to as an “R-PDCCH.”
In the LTE (Long Term Evolution) system, a DL grant (also referred to as “DL assignment”), which indicates a downlink (DL) data assignment, and a UL grant, which indicates an uplink (UL) data assignment are transmitted through a PDCCH. The DL grant indicates to the terminal that a resource in the subframe in which the DL grant is transmitted has been allocated to the terminal. On the other hand, the UL grant indicates to the terminal that a resource in a target subframe which is predetermined by the UL grant has been allocated to the terminal.
In the LTE-Advanced system, a region (R-PDCCH for relay station (relay PDCCH) region) in which channel control signals for relay stations are mapped is provided in the data region. Similarly to the PDCCH, a DL grant and UL grant are mapped to the R-PDCCH. In the R-PDCCH, the DL grant is mapped in the first slot and the UL grant is mapped in the second slot (refer to Non-Patent Literature “hereunder abbreviated as NPL” 1). Mapping the DL grant only in the first slot reduces a delay in decoding the DL grant, and allows relay stations to prepare for ACK/NACK transmission for DL data (transmitted in the fourth subframe following reception of the DL grant in FDD). Thus, each relay station monitors channel control signals transmitted using an R-PDCCH from a base station within a resource region indicated by higher layer signaling from the base station (i.e., a “search space”) and thereby finds the channel control signal intended for the corresponding relay station.
In this case, the base station indicates the search space corresponding to the R-PDCCH to the relay station by higher layer signaling.
In the LTE and LTE-Advanced systems, one RB (resource block) has 12 subcarriers in the frequency domain and has a width of 0.5 msec in the time domain. A unit in which two RBs are combined in the time domain is referred to as an RB pair (for example, see FIG. 1). That is, an RB pair has 12 subcarriers in the frequency domain, and has a width of 1 msec in the time domain. When an RB pair represents a group of 12 subcarriers on the frequency axis, the RB pair may be referred to as simply “RB.” In addition, in a physical layer, an RB pair is also referred to as a PRB pair (physical RB pair). A resource element (RE) is a unit defined by a single subcarrier and a single OFDM symbol (see FIG. 1).
Further, when the PDSCH is allocated to the RB, RBs may be allocated in units of RBs or in units of RBGs (Resource Block Group). An RBG is a unit in which a plurality of adjacent RBs are arranged. Further, the RBG size is defined by a bandwidth of a communication system, and LTE has 1, 2, 3 and 4 as the defined RBG size.
A PDCCH and R-PDCCH have four aggregation levels, i.e., levels 1, 2, 4, and 8 (for example, see NPL 1). Levels 1, 2, 4, and 8 have six, six, two, and two “mapping candidates,” respectively. As used herein, the term “mapping candidate” refers to a candidate region in which a control signal is to be mapped, and a search space is formed by a plurality of mapping candidates. When a single aggregation level is configured for a single terminal, a control signal is actually mapped in one of the plurality of mapping candidates of the aggregation level. FIG. 2 illustrates an example of search spaces corresponding to an R-PDCCH. The ovals represent search spaces for the aggregation levels. The multiple mapping candidates in each search space for each aggregation level are located in a consecutive manner on VRBs (virtual resource blocks). The resource region candidates in the VRBs are mapped to PRBs (physical resource blocks) through higher layer signaling.
Studies are being conducted with respect to individually configuring search spaces corresponding to the ePDCCHs for terminals. Further, with respect to the design of the ePDCCHs, part of the design of the R-PDCCH described above can be used, and a design that is completely different from the R-PDCCH design can also be adopted. In fact, studies are also being conducted with regard to making the design of the ePDCCHs and the design of R-PDCCHs different from each other.
As described above, a DL grant is mapped to the first slot and a UL grant is mapped to the second slot in an R-PDCCH region. That is, a resource to which the DL grant is mapped and a resource to which the UL grant is mapped are divided on the time axis. In contrast, for the ePDCCHs, studies are being conducted with regard to dividing resources to which DL grants are mapped and UL grants are mapped on the frequency axis (that is, subcarriers or PRB pairs), and with regard to dividing REs within an RB pair into a plurality of groups.
Further, the LTE-Advanced system supports carrier aggregation (CA). CA is a new function introduced in the LTE-Advanced system, which bundles a plurality of system bands termed component carriers (CCs) in LTE, thereby realizing an improvement in a maximum transmission rate (See NPL 2). When a terminal uses a plurality of CCs, one CC is configured as a primary cell (PCell) and a remaining CC is configured as a secondary cell (SCell). The configuration of the PCell and SCell may vary for each terminal.
Further, a resource allocation method termed “cross-carrier scheduling” which performs an inter-cell interference control in units of CCs in PDCCH has been introduced in the LTE-Advanced system. In cross carrier scheduling, a base station can transmit DL grants and UL grants for other CCs in the PDCCH region of the CC having good channel quality (for example, see FIG. 3B). If cross carrier scheduling is adopted, a PDCCH is transmitted from different a CC between adjacent cells, thereby allowing the inter-cell interference of PDCCH to be reduced.
In cross carrier scheduling, since resource allocation information is transmitted for each CC, the PDCCH increases in proportion to the number of allocated CCs. Therefore, as the number of CCs increases, search spaces are overlapped between different terminals, and thus the probability of blocking (collision) increases. Furthermore, there is a possibility that blocking occurs not only between different terminals but also between PDCCHs of different CCs intended for a single terminal. The blocking between PDCCHs of the single terminal limits the number of CCs that can be simultaneously allocated to the same terminal and limits a maximum transmission rate for each terminal. Therefore, in PDCCHs of the LTE-Advanced system, a method is adopted in which at the time of calculating a search space, consecutive CCE regions different from each other are configured as search spaces for CCs by using CIF (Carrier Indication Field) given to each of the CCs, in addition to UE IDs.
In addition, “localized allocation” which allocates ePDCCHs collectively at positions close to each other on the frequency band, and “distributed allocation” which allocates the ePDCCHs by distributing ePDCCHs on the frequency band have been studied as allocation methods for ePDCCHs (for example, see FIG. 4). The localized allocation is an allocation method for obtaining a frequency scheduling gain, and can be used to allocate an ePDCCH to a resource that has favorable channel quality based on channel quality information. The distributed allocation distributes ePDCCHs on the frequency axis, and can obtain a frequency diversity gain. In the LTE-Advanced system, both a search space for localized allocation and a search space for distributed allocation may be configured (for example, see FIG. 4).
Furthermore, dividing each PRB pair into a plurality of resources in an ePDCCH has been studied. Resources obtained by dividing the PRB pair may be referred to as eCCEs (enhanced control channel elements) or eREGs (enhanced resource element groups). In addition, in the following description, eCCEs may be simply referred to as “CCEs.” The number of REs forming one CCE in a PDCCH is fixedly configured to 36 REs, but the number of REs forming one CCE in an ePDCCH varies depending on a division method. As the division method, a division method in units of subcarriers or a division method by generating resource (RE) groups have been studied. FIG. 5 illustrates an example in which a plurality of PRB pairs are configured as search spaces for ePDCCHs and each PRB pair is divided into four CCEs in units of subcarriers. In FIG. 5, CCEs obtained by dividing each PRB pair are referred to as CCE#(4N), CCE#(4N+1), CCE#(4N+2), CCE#(4N+3), respectively (where, N=0, 1, 2, and 3).